A bicycle is normally provided with a rear derailleur active on a groupset, which consists of a series of coaxial toothed wheels (sprockets) of different diameters and number of teeth coupled with the hub of the rear wheel.
The derailleur engages a transmission chain extending in a closed loop between the groupset and the crankset, moving it on toothed wheels having different diameter and number of teeth, so as to obtain different gear ratios.
In particular, downward gearshifting is said when the chain passes from a toothed wheel of larger diameter to a toothed wheel of smaller diameter, and upward gearshifting is said when the chain moves from a toothed wheel of smaller diameter to a toothed wheel of larger diameter. Concerning this, it should be noted that with reference to the rear derailleur, downward gearshifting corresponds to the passage to a higher gear ratio and upward gearshifting corresponds to the passage to a lower gear ratio.
The movement in the two directions of the rear derailleur is obtained through an actuation device mounted on the handlebars so as to be easy for the cyclist to maneuver.
More in particular, in a mechanical gearshift, the rear derailleur is moved between the toothed wheels of the groupset, in a first direction by a traction action exerted by an inextensible control cable that is normally sheathed (commonly called Bowden cable), in a second opposite direction by the release of the traction of the cable and by the elastic return action of a spring provided in the derailleur itself.
The movement of the rear derailleur is carried out according to an articulated parallelogram linkage wherein the sides of the parallelogram are articulated in pairs, along respective substantially parallel rotation axes, through pins. In particular, such an articulated parallelogram is formed from a support body, intended to remain fixed with respect to the frame, a so-called chain-guide adapted for moving the chain between different engagement positions on the toothed wheels, and a pair of articulation arms or connecting rods (generally identified as outer connecting rod and inner connecting rod), which connect the chain-guide in a mobile manner to the support body.
The traction of the control cable opposes the action of an elastic spring active in the rear derailleur that tends to push the connecting rods of the derailleur towards the smaller gear of the groupset, whereas the release of the control cable frees the elastic energy of such a spring.
Therefore, the traction or the release of the control cable determine respective rotations of the connecting rods of the derailleur with the consequent movement of the chain-guide that faces makes the chain face the desired toothed wheel for precise gearshifting.
Normally, the direction in which the movement is determined by the release of the traction of the cable and by the return spring is that of downward gearshifting; vice-versa, the traction action of the control cable takes place in the direction of upward gearshifting, wherein the chain moves from a wheel of smaller diameter to a wheel of larger diameter.
In the actuation device, the control cable is actuated in traction or in release through winding and unwinding on a rotor element, commonly called cable-winding bush, the rotation of which is carried out by the cyclist with a suitable control lever, or with two control levers (a first lever for upward gearshifting and a second lever for downward gearshifting).
In any case, the actuation device must provide for the cable-winding bush to be kept stationary in rotation in a number of predetermined angular positions, spaced apart by predetermined indexing angles and corresponding to the different positions of the derailleur required by the different ratios, namely on the different toothed wheels of the groupset. This function is obtained with the so-called indexers, many types of which are known in the field, variously active between the cable-winding bush and the fixed casing of the device.
Examples of such indexers can be found in documents U.S. Pat. No. 6,216,553, U.S. Pat. No. 5,676,020, EP2527240 and U.S. Pat. No. 6,484,603.
During downward gearshifting, the control cable is released by an amount such as to allow the indexer to stop the rotation of the cable-winding bush at the indexing angle necessary to position the transmission chain exactly on the preselected toothed crown of the groupset, so as to allow precise and reliable gearshifting.
The Applicant has noted that, although precise and reliable, downward gearshifting is not always prompt, in other words it is not reactive and immediate, since the release of the control cable has a certain inertia in activating the elastic spring of the rear derailleur that moves the connecting rods of the derailleur.
The Applicant has indeed noted that in order to carry out downward gearshifting it is necessary for the slack of the control cable to propagate completely to the elastic spring of the rear derailleur, so that the elastic energy freed by it moves the connecting rods by the amount necessary to reposition the chain-guide at the toothed wheel to be engaged. The complete propagation of such slack is not instantaneous but is subject to the mechanical inertia of the linkage of the gearshift.
The Applicant has perceived that an extra stroke of the control cable during downward gearshifting would allow much more reactive downward gearshifting, since the rear derailleur would move to the toothed wheel to be engaged before the slack of the control cable has completely propagated to the derailleur itself.
The Applicant has, however, found that in this way, when the slack of the control cable has completely propagated to the rear derailleur, the rear derailleur would position the transmission chain not perfectly at the toothed wheel to be engaged but in an intermediate position between the toothed crown to be engaged and the immediately smaller one with consequent instability of gearshifting.